The overall goal of this project is a deeper understanding of the role of demyelination in loss of function following traumatic injury to the spinal cored. Its medial significance lies in the importance of knowing the mechanisms of injury and recovery as a first step in developing therapy for this devastating currently untreatment condition. Such therapy might involve improvement of remyelination or pharmacological reversal of action potential conduction failure. The study will also continue to provide basic information on the biophysical organization of myelinated axon, and on the more general phenomenon of demyelination pathophysiology in the mammalian nervous system. A new model of spinal cord compression injury in adult guinea pigs will be quantified, morphometrically, electrophysiologically and functionally. This preparation has been developed as a result of experience with cat spinal cord weight-drop contusion injury, and it provides substantial advantages for studying chronic injuries, particularly for in vitro electrophysiology. It also demonstrates a unique advantage in that there is a clear separation, at the level of behavioral function, between acute and delayed damage to the spinal cord. It will be used specifically to examine differences in myelinated axon structure and physiology between animals which recover function and those which do not, following similar primary injuries. The effect of lesion length on remyelination and chronic conduction deficits will also be investigated. This is an essential but neglected issue in modeling clinical trauma, where lesions are typically extensive. Intracellular recordings will be made from axons in spinal cord tracts isolated from animals at a succession of stages after experimental spinal cord injury. The quantitative morphometry of the lesion will be examined in 1 micron plastic sections, using line sampling techniques. A particular concern will be to identify structural and physiological differences between axons that respond and those that do not respond to the application of 4-aminopyridine (4-AP), a potassium channel blocker that has been shown, during the first grant period, to restore conduction in some axons of chronically injured cords. Intracellular injection of horseradish peroxidase (HRP) will be used to allow correlation of structure and function in individual fibers. The conduction characteristics of axons, their temperature of conduction block, and their responses to extracellular application of 4-AP, will be measured prior to injection of HRP. Subsequently, these axons will be identified in vibratome sections and embedded in plastic for morphometric studies using semi-thin and thin sections.